1. Technical Field
The present invention relates to an optical reproduction device, an optical recording/reproduction device and an optical reproduction method.
2. Related Art
In recent years, a coaxial recording system (co-linear system) has been proposed as a holographic memory recording/reproduction system. As compared with a conventional two-beam interference system, the co-linear system has the advantages such that the optical system can be greatly simplified, the co-linear system is strong with respect to external disturbances such as vibrations, and introduction of a servo mechanism is easy. In this co-linear system, signal light and reference light, that are modulated and generated by a spatial light modulator, are collected by a same lens and with their optical axes being in common, and the interference fringes (diffraction grating) formed by the interference between the signal light and the reference light are recorded on an optical recording medium as a hologram.
By displaying a signal light pattern, that two-dimensionally encodes digital data, on a spatial light modulator, the digital data is superimposed on the signal light. By irradiating reference light as reading light onto an optical recording medium on which a hologram is recorded, the signal light is reproduced from the recorded hologram. The superimposed digital data can be decoded from the reproduced signal light.
On the other hand, there is proposed a reproduction method (hereinafter called the “negative/positive reproduction method”) in which, by adding a DC component to reproduced light at the time of reproduction of a hologram, two reproduced images that are a negative image and a positive image are generated from one hologram, and by determining the difference between the both, the signal-to-noise ratio (SNR) is improved. Further, there is proposed a technique (hereinafter called the “coherent addition method”) in which recording is carried out by providing a phase difference of 0 and π to the on pixels by using two spatial light modulators in order to provide a phase difference, and at the time of reproduction, by adding a DC component of a phase difference of π/2 and causing interference, the interference between the pixels of the reproduced data page is made to have a linear characteristic, and the SNR is improved.
All of these techniques are to improve the SNR, and all of these techniques cause interference between the high-order component of the reproduced diffraction light and the DC component added at the time of reproduction, and obtain a new reproduced image. The DC component that is added is generated in the signal light region of the spatial light modulator that is a liquid crystal display (LCD) or the like. However, it is difficult to control at the same time both the phase and the amplitude (light intensity) of the “added DC component” merely by a spatial light modulator. Usually, controlling of the phase of the “added DC component” is carried out by a spatial light modulator. Accordingly, a structure that, independently of the phase, controls the amplitude of the “added DC component” is required separately from the spatial light modulator.
Further, in a form in which light from a light source is made incident as is onto a spatial light modulator at which a signal light region and a reference light region are formed, when reference light is irradiated as reading light at the time of reproduction, light leaks out also from the signal light region of the spatial light modulator. For example, when the precision of the liquid crystal elements that are disposed as a transmission-type spatial light modulator is low, there are cases in which the light that is transmitted through the off pixels at the displayed position of the signal light pattern becomes unnecessary leakage light and is detected at a light detector, and generates noise with respect to the reproduced signal light.